Reaction mechanisms in hydrated magnesium silicate glass investigated by Ab-Initio methods and Metadynamics

In the field of magnesium-containing silicate glass alteration under aqueous conditions, enstatite glass (MgSiO3) is used as a valuable system to investigate reaction pathways occurring in its hydrated form by means of Ab-Initio methods. In contrast to previous dissolution studies in which the water molecules interacted only at the glass surface or with small molecular clusters, here we follow reaction pathways for water molecules introduced in the voids of a realistic glass structure. Metadynamics combined with atomic charges derived from electrostatic potential suggest that water dissociation follows a two-step scheme: at first, the water molecule interacts with a SiO4 or MgOx(x >= 3) structural unit via dipole-dipole or dipole-charge interactions, and second, a proton transfer occurs. It also appears that SiOH-Mg hydrolysis reactions are the more likely to occur. Furthermore, it is inferred that pathways exist for complete H2O dissociation in two H+ and one O2- ions, leading to new bonding between the free oxygen ion and Si or Mg cations forming the network. This phenomenon could probably counterbalance hydrolysis and depolymerization by increasing again network connectivity. Several bond dissociation energies are evaluated and the Mg-O bond dissociation enthalpy from the Mg-OHSi linkage is derived to be 1.34 eV (or 128.84 kJ mol-1).